Why am I getting "Origin is not allowed by Access-Control-Allow-Origin" from the same domain?

Hi all. I have swagger-editor hosted at my hosting provider, accessible from mydomain/API.

The editor functions fine, but when I try to execute any of the queries in the preview pane I get

Origin http://mydomain.com is not allowed by Access-Control-Allow-Origin

Before I investigate enabling CORS at my host, can anyone tell me why it's complaining about something it thinks originates from its own domain? I understand that my actual API definition (from the editor) is stored locally under the browser's local storage; does that have anything to do with it?